The idea of pulsating DC electric current through a sheet metal blank that is clamped to a surface in targeted shape to eliminate spring-back was derived from observations of tensile tests conducted in conjunction with superimposed pulses of DC current. The idea of improving formability of sheet metal parts made of aluminum alloys by pulsating DC current resulted from this testing. This concept was disclosed in S. Golovashchenko, A. Krause, J. Roth “Method and apparatus for forming a blank as a portion of the blank receives pulses of direct current” U.S. Pat. No. 7,516,640. It was observed while analyzing the stress-strain tensile test curve that the stress is reduced when the current is propagated through the blank.
Applicant is the author of a defensive publication entitled “Calibration of shape distortion of stamped or heat treated parts by propagating pulses of DC current” Defensive publication IPCOM 000167243D. The defensive publication discloses that after a stamped blank is released from a stamping die, it often changes shape as a result of spring-back. The shape of the sheet blank can be corrected by propagating a DC current through it.
Weight reduction requirements for vehicles are leading to broader adoption of parts made from advanced high strength steels and aluminum alloys. Spring-back for these materials is more difficult to compensate for than spring-back in mild steels. To develop a die for aluminum sheet metal or high strength steels that compensates for spring-back may require, in some cases, ten or more die face recuts.
Recently, a number of numerical algorithms of spring-back compensation were developed to calculate the die face geometry. Another approach to resolve the spring-back issue is to stretch the blank at the end of the stamping cycle to reduce the bending moments inside the blank. However, this approach limits the depth of the drawing process, which is already restricted by insufficient formability of advanced high strength steels and aluminum alloys. The use of algorithms and stretching the blank post-forming are not satisfactory if the material properties vary from coil to coil, or if the die geometry varies during its life time due to the die wear.
Hydro-formed members cannot be stretched like sheet metal parts to eliminate spring-back. The primary approach to eliminating spring-back in hydro-formed tubular members is to over bend the tube to compensate for spring-back. However, this approach is subject to manufacturing variability due to variations in material content, material thickness, and other factors.
The idea of calibrating blanks stamped from sheet metal by clamping them to the targeted shape and propagating pulsed current induced through high voltage discharge of capacitors through the insulated metallic coil clamped to the sheet metal blank was presented in U.S. Pat. No. 7,540,180; S. Golovashchenko, V. Dmitriev, P. Canfield, A. Krause, C. Maranville “Apparatus for electromagnetic forming with durability and efficiency enhancements.” This concept was also explained in S. Golovashchenko: “Springback calibration using pulsed electromagnetic field,” Proceedings of the 6th International Conference NUMISHEET 2005, Detroit, p. 284-285.
All the efforts described above target calibration of blanks stamped from sheet metal. Problems relating to calibrating and relieving stress in hydro-formed aluminum or high strength steel tubular parts were not disclosed or addressed in the above publications and patents.